Safety considerations make it very important that hydraulic brake systems and hydraulic steering apparatus in vehicles be supplied with pressurized hydraulic fluid under conditions such that reliable functioning is assured under all operating conditions. Nevertheless, it is not unusual for even simple hydraulic systems having only a single supply pump to be relied upon additionally to power at least one auxiliary consuming unit, e.g. a piston-and-cylinder device.
In order to ensure reliable powering of the brake system and the steering apparatus on a priority basis, it is common to operate the hydraulic system with the individual consuming units being switched among each other, and being protected from each other's failures by so-called "priority valves".
This customary type of control gives rise to disadvantages. A plurality of priority valves must be laid out in such a way that their response behavior must be well coordinated. Moreover, the volumetric flow range of the fluid extends up to the maximum output flow of the common hydraulic pump. The existence of this boundary condition has heretofore made it difficult to optimize the control circuits with regard to response behavior and space requirements. Consequently, with the above approach it would be necessary in individual cases to make sacrifices in terms of substantial flow losses, i.e., energy losses, in the control circuits.